This invention relates generally to a sample inlet system for an electron capture detector for use with high resolution capillary columns in gas chromatography and more in particular to a method of minimizing the sample contact with the metal walls of the detector cell and the dilution of the sample with the make-up gas.
By the electron capture detection technique in gas chromatography, a tritium or Ni.sup.63 source ionizes the molecules of a carrier of make-up gas as it flows through the detector and the slow electrons thus produced are caused to migrate to the anode, forming a steady or pulsed current. This current becomes reduced if a sample containing electron absorbing molecules is introduced and this loss of current can be amplified by an electrometer for analysis.
The electron capture detector is extremely sensitive to certain molecules such as alkyl halides, but is relatively insensitive to hydrocarbons, alcohols, ketones, etc. This selective sensitivity to halides makes the detection method especially valuable for the trace analysis of many environmentally important organic compounds such as pesticides. There is shown in FIG. 1 the general design of a prior art electron capture detection system such as the commercially available one disclosed by P. L. Patterson in J. Chromatogr., 134 (1977) at page 25). The top portion of a gas chromatography column 11 through which the sample to be analyzed is led nto the detector is housed concentrically inside an inlet tube 12 so as to form a passageway 13 having an annular cross section between the inner wall of the inlet tube 12 and the outer wall of the column 11. This passageway 13 is for a make-up gas, the use of which may become necessary in order to push the column gas (sample with a carrier gas) into the detector, for example, when the column 11 is a capillary column. The make-up gas then becomes mixed with the gas from the column 11. A generally cylindrical metal anode 15 is connected to the upper end of the inlet tube 12, separated therefrom by a ceramic insulator 16. The other end of the anode 15 opens into a cylindrical cell 20 (of length L and diameter D), separated therefrom by another ceramic insulator 21. The top end of the cylindrical anode 15 is provided with side ports 22. Thus, the sample from the column 11 and the make-up gas from the passageway 13 become mixed together as they travel upwards through the cylindrical anode 15, entering the interior of the cell 20 from below, some of this mixed gas passing through the side ports 22. On the inner wall of the cell 20 is a radioactive foil 25 which, for example, may be a Ni.sup.63 or H.sup.3 source. The top of the cell 20 is connected to an exit tube 26.
The prior art electron capture detector of FIG. 1 has several disadvantages. Firstly, because the sample from the column 11 is made to pass through the metal anode 15 before entering the detector cell 20, there results a sample loss by adsorption and this can cause chromatographic peak broadening. Secondly, a sample loss of adsorption occurs also on surfaces within the cell 20 especially when they are activated by hydrogen. Even when hydrogen is not used as the carrier gas, the presence of hot metal or ceramic surfaces with which the sample can come in contact should be expected to have detrimental effects. Thirdly, the make-up gas, when its use is necessary, tends to dilute the sample, decreasing the sensitivity of the detector. Fourthly, the detector cell 20, according to the prior art design as shown, includes regions at the top corners which are not actively swept by the carrier gas. An electron capture detector is generally sensitive to oxygen, and it is therefore necessary to prevent its back diffusion by increasing the length-to-diameter ratio of the exit tube 26. This necessarily tends to enlarge such unswept areas especially when the length-to-diameter ratio (L/D) of the cell 20 is decreased.
For the above and other reasons, electron capture detectors have not been used extensively in conjunction with high resolution capillary columns.
It is therefore a general object of this invention to provide an electron capture detector with a sample inlet system which can reduce the dilution of the sample with the make-up gas which may have to be used.
It is another object of the present invention to provide a small volume electron capture detector cell wherein the mixing volume effects are minimized or eliminated.
It is a further object of the present invention to provide an electron capture detector with a sample inlet system which does not require the sample to pass through a metal anode before entering the active volume of the cell.